We examine the mechanism of ultrafast internal conversion between the B band (Soret band) and the Q band in porphine (H₂P), the prototypical free-base porphyrin, using electronic structure studies and on-the-fly surface-hopping nonadiabatic dynamics. Our study highlights the crucial role of dark states within the N band which are found to mediate B/Q state transfer, necessitating a treatment beyond Gouterman's classic four-orbital model. The sequential B → N → Q pathway dominates largely over the direct B → Q pathway which is found to be energetically unfavorable. Potential energy surface cuts and conical intersections between excited states are determined by TDDFT and validated by CASSCF/CASPT2 and XMCQDPT2 calculations. Both the static analysis and on-the-fly surface-hopping calculations suggest a pathway which involves minor structural deformations via in-plane vibrations. The B → N conversion is a barrierless adiabatic process occurring within ∼20 fs, while the subsequent N → Q conversion occurs via a conical intersection within ∼100 fs, in agreement with time-resolved experiments for porphine and related free base porphyrins. Furthermore, evidence for both sequential and direct transfer to the Q_x and Q_y states is obtained.

中文翻译：

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游离卟啉的超快光化学：暗态介导的B→Q内部转化的理论研究†

我们研究了吗啡（H ₂）的B波段（Soret波段）和Q波段之间的超快内部转化机理。P），典型的游离碱卟啉，利用电子结构研究和动态的表面跳跃非绝热动力学进行研究。我们的研究强调了N波段内暗态的重要作用，这些暗态可以介导B / Q态转移，因此有必要超越Gouterman的经典四轨道模型进行处理。顺序的B→N→Q路径在直接B→Q路径上占主导地位，而直接B→Q路径在能量上不利。势能表面切割和激发态之间的圆锥形交点由TDDFT确定，并由CASSCF / CASPT2和XMCQDPT2计算验证。静态分析和飞行中的表面跳变计算均提出了一条路径，该路径涉及通过平面内振动。B→N转化是在约20 fs内发生的无障碍绝热过程，随后的N→Q转化是通过约100 fs内的圆锥形交点发生的，这与对吗啡和相关游离碱卟啉的时间分辨实验一致。此外，获得了连续转移和直接转移到Q _x和Q _y状态的证据。